(a) Field of the Invention
The present invention relates to a pulse power generator using a semiconductor switch, and more particularly to such a pulse power generator using a semiconductor switch, which enables its lifespan to be significantly improved, allows for its miniaturization, and makes it possible to diversely control a high-voltage pulse output finally.
(b) Background Art
In general, a high-voltage pulse generating circuit uses various test equipment and a plasma generator (PSII, etc.) as a load. A conventional high-voltage pulse generating circuit embraces lots of problems in terms of lifespan of the switch, variation in the pulse width, increase in the operating frequency, adjustment of the pulse voltage, need for a high-voltage DC power supply and the like.
For example, such a conventional pulse generating circuit is divided into a type employing a Marx generator using two-electrode spark gaps, a type employing a vacuum-tube switch and a type in which a low-voltage pulse is boosted to a high-voltage pulse simply using a pulse transformer. However, the type employing the spark gaps or the vacuum-tube switch encounters shortcomings in that its lifespan is short, the adjustment of a pulse width is not flexible and very difficult, there is a limitation in increasing a pulse repetition rate and a high-voltage DC power supply circuit is needed. Also, the type employing the pulse transformer entails demerits in that it is difficult to achieve an ultra fast rise time of a pulse due to the inductance of the transformer, the circuit is complicated since a reset circuit must be installed additionally due to the magnetic saturation of a transformer core, a noise is generated and an increase of the pulse width is difficult.
Now, the conventional prior art will be described hereinafter in detail with reference to the accompanying drawings.
FIG. 1 is a circuit diagram illustrating the construction of a Marx generator using spark gaps as disclosed in U.S. Pat. No. 4,900,947 issued to Maurice Weiner, Ocean, et al.
The Marx generator shown in FIG. 1 is a high-voltage pulse generator of a type which is most widely used in a high power field.
As well known in the art, it is required that a pulse generating circuit should be supplied with a DC power to allow capacitors to be charged in parallel and the capacitors should be connected with one another in series at a specific time point to discharge their voltage for application to a final output terminal thereby to generate and output a high-voltage pulse. Thus, as shown in FIG. 1, the Marx generator includes a plurality of capacitors and a plurality of spark gap switches. When DC power is applied to an input terminal of the Marx generator through the spar gap switches, the capacitors are charged in parallel and then are connected with one another in series to discharge their voltage by actuating the spark gap switches at a specific time point.
That is, the plurality of capacitors are connected to one another in parallel via resistors with respect to a high-voltage DC power to form a bank, and a spark gap switch is mounted between a positive (+) terminal of each capacitor and a negative (−) terminal of a next capacitor. When high-voltage DC power is applied to an input terminal of the Marx generator, the respective capacitors are charged in parallel via the resistors. Also, when the spark gap switches are turned on to be electrically conducted along with the discharging of the generator at a desired specific time point, the DC voltage charged in the respective capacitors is instantaneously discharged simultaneously. At this time, a final high-voltage DC power obtained by adding the DC voltages of the respective capacitors discharged in series is applied through a final output terminal.
However, it is necessarily required that a high-voltage DC power supply should be basically installed at an input terminal of the generator so as to operate the spark gap switches and a particular trigger circuit is needed to correctly adjust the time point when a pulse voltage is generated. Especially, the Marx generator has several demerits in that a maximum pulse repetition rate (pulse frequency) is restricted, the arbitrary adjustment of a pulse width is impossible, and the spark gap switches have a great limitation in terms of lifespan due to abrasion generated each time a spark occurs as being mechanical discharge switches. In addition, a short circuit occurs in a load, it is impossible to restrict a short circuit current. Further, the Marx generator has problems in that the smaller the number of stages being connected is, the higher the voltage each stage must endure becomes so that a withstanding voltage of each switch is raised, and in that when the withstanding voltage of each stage is alternatively lowered, the number of stages to be connected increases so as to obtain a necessary voltage.
Meanwhile, there have been attempts to utilize insulated gate bipolar transistors (hereinafter abbreviated as “IGBT”) as semiconductor switches instead of the spark gap switches in the Marx pulse generator shown in FIG. 1. The GBT has a permanent lifespan. In case where the GBT is used, the disadvantages of the conventional Marx pulse generator are overcome so that, for example, a pulse repetition rate and a pulse width can be controlled. But such a conventional Marx generator still has a risk that its reliability may be deteriorated due to strict restraints on the driving of the switches simultaneously and even voltage distribution of switches. The higher the voltage is, the more the number of stages increases so that the size of the system also increases.
The most critical technology in the pulse generator using the IGBT as the semiconductor switch is to overcome the voltage rating and the current rating of the semiconductor switch. The IGBT has a low current rating and a low voltage rating unlike an existing gas discharge switch. A method may be employed in which one IGBT is not used instead of a single spark gap switch, but a plurality of IGBTs are be connected with one another in series as many as needed so as to be sufficient to withstand a voltage rating so that they are turned on/off concurrently. However, in this case, when the IGBTs are turned on or off, a unbalance of the voltage is prone to occur due to a difference in driving timing. At this time, any voltage higher than the voltage rating of IGBT may cause the IGBT to be damaged immediately. Actually, despite application of a completely synchronized gate signal, it is impossible to concurrently turn on/off the IGBTs due to the difference of internal parameters (for example, resistor value or inductance value) of individual elements. If the turning on/off of the IGBT is not synchronized, for example, if only one IGBT from multi connected IGBTs is not synchronized with others to cause turned off earlier, the entire voltage is applied across the IGBT which is not synchronized to cause the IGBT to be damaged, which results in sequential breakages of the remaining IGBTs due to the damage of the specific IGBT. Moreover, when the IGBTs are driven in series, each switch needs an independent gate power supply. In this case, as it goes toward an upper portion in a series-switch arrangement, the dielectric strength of the independent gate power supply must increase. Thus, in a high-voltage driving, one of the most difficult technologies is known as high voltage insulation of the gate power supply.
As another example of a technology employing the IGBT in the art, in FIG. 2, there is shown a power modulator as disclosed in U.S. Pat. No. 5,905,646 issued to Walter Frederick John Crewson, et al. The power modulator employs the IGBTs and transistors (hereinafter abbreviated as “TR”), in which voltage of a primary winding of a transformer is amplified through the transformer.
In the meantime, both the aforementioned Marx pulse generator and the power modulator using the IGBTs and the TRs as shown in FIG. 2 employ a high-voltage charger to which an SCR control method is applied. A conventional high-voltage charger, which has been used so far, entails a problem in that its entire size is greatly large. Therefore, there is a need for an improved high-voltage charger.
A gate power generator adopts a high-voltage insulation (double insulation) method, and an optical signal using an optical driving gate circuit may be used as a gate signal. The gate power generator, which has been developed so far, is very complicated in a structure for achieving a high-voltage insulation since it is subjected to a multi-staged voltage transformation for the sake of the high-voltage insulation. In addition, the gate power and the gate signal are generated by means of a separate construction, respective, which leads to a complexity of the entire construction. There is therefore a need for an improvement associated with the generation of the gate power and the gate signal.
Besides the problems which have been known so far in regard to the Marx pulse generator of FIG. 1 and the power modulator of FIG. 2, there are additional problems in that the two types have a limitation in a pulse width (<10 μs). Particularly, the generator of the type employing the TRs has a great restriction in a pulse rise/fall time due to a leakage inductance. Also, the generator embraces a problem in that the overall size of the apparatus is large and its operating efficiency is low. In addition, the generator of the type employing the IGBTs and the TRs enables protection of generation of arcs, but is problematic in that circuits are complicated.